The prior art, and in particular French Pat. No. 1 575 984 teaches that macromolecular compounds of the type consisting of a main hydrocarbon chain upon which are grafted relatively long straight hydrocarbon chains, i.e. of at least 10 or 12 carbon atoms, can interfere with the crystallization of heavy paraffins.
This property is advantageously utilized in order to improve the stability of paraffin-rich crude oils, either by preventing the formation of scale deposits during the extraction or transportation of crude petroleum oils, or by preventing the accumulation of paraffin crystals in slack areas which thereby limit fluid flow at low temperature.
In macromolecules of molecular weights of at least 10,000 and having lateral chains which are connected to central chain through intermediary functional groups, it appears that the most interesting groups are by far, the ester groups and especially the amide groups.
The preparation of amide linked derivatives can be envisaged by polymerization of N-alkylamide containing monomers of unsaturated carboxylic acids, or by amidation of prepolymerized structures from unsaturated carboxylic acids.
The first method involves the problem of manufacturing unsaturated N-alkylamide containing monomers. Indeed, the reaction that comes to mind is condensation of an unsaturated acid, for example acrylic acid, with a fatty amine.
In practice, this reaction is totally in competition with the Michael reaction and instead of leading to N-acrylamide, alkylaminopropionic acid is obtained. The solutions that are found in the literature are complex, and consist for example, in condensation of an amine with an acrylic acid derivative, 2-carboxymethyl-7-oxa(1,2,2)bicyclo-5-hexene and dissociation of the amide obtained in acrylamide and furane (EP 104 584), or condensation of the amine on acryloxyl chloride, such as cited in EP 115 703.
The other method encounters another sort of difficulty. The polymerization derivative of the unsaturated carboxylic acid is poorly adapted to the reaction with a long chain amine, due to the incompatibility of the two reactants, polyacrylic acid or homologue, which is of totally hydrophilic nature, and alkylamine which is of totally oleophilic nature. Of course, instead of a carboxylic acid polymer, it is possible to use copolymers, for example, a copolymer of an unsaturated carboxylic acid and an olefin, even a terpolymer of unsaturated monocarboxylic acid, unsaturated .alpha.-.beta.-dicarboxylic acid and olefin such as taught by FP 82 09 908. These compounds contain monomers endowed with very different reactivities and the result of the copolymerization is strongly influenced by operating conditions, from which stem uncertainties as to the structure and the reproductibility of the amide-based compounds derived therefrom. But, above all, the molecular weights obtained by these methods are too low, and their amide derivatives function as dispersants of the various particles than can be present in oils, but do not act in syncrystallization with the paraffins which are responsible for undesirably high pour points.
The applicants have found that it is possible to very easily realize N-amide based polymer compounds by reacting, in aqueous heterogeneous medium, a polyacrylic acid and a long chain fatty amine and that the products thus obtained offer a very high activity of paraffin deposit retention and of depression of the pour point in the crudes, that this activity was only very slightly influenced by the origine of the crude, and that the obtention of the products and their remarkable properties was very easy to reproduce.
The principle of obtaining products according to the invention is described herein-below:
The apparently very heterogeneous mixture of polyacrylic acid (APA) in on aqueous solution of fatty amine, preferably aromatic hydrocarbonbased solvent and a dehydratation catalyst is heated without stirring, and the raise in temperature is conducted in such a manner so as to obtain the homogenization of the reaction medium, then both the water carried along by the diluted polyacrylic acid charge and that appearing from the amidation reaction is eliminated by azeotropic stripping.
The polyacrylic acid can be prepared from the polymerization of acrylic acid initiated by a generator of free radicals such as azoisobutyronitrile (AIBN) utilized jointly with a growth limiter of the chains, in the present case, thioglycolic acid (TGA) or again by a redox system such as the potassium persulfate/metabisulfite system. Its molecular weight should be comprised between 5,000 and 500,000 but an acid of 80,000 to 150,000 is preferable, and is perhaps obtained more easily by redox initiation.
The fatty amine will preferably be a long chain monoalkylamine, containing at least 12 carbon atoms, and more particularly C.sub.18 amines such as alkylamine or stearylamine, and even more preferably C.sub.20 /C.sub.22 amines such as erucic amine or behenic amine.
The dehydratation agent is optional but industrial catalysts will advantageously be utilized such as zinc oxide or boric acid. The choice of solvent is not particularly limitative. It is however necessary on the one hand that its water stripping power due to azeotropy be effective and on the other hand that its boiling point not be too low.
Toluene, xylene, industrial aromatic solvents having boiling points lower than 300.degree. C. or mixtures of such solvents are particularly suitable for this purpose. They have the supplementary advantage of being good solvents of macromolecular compounds according to the invention and of being soluble in crude petroleum oils. They can thus constitute the bases of formulations of rheological and anti-deposit additives. Such formulations preferably contain from 20 to 80% by weight of macropolymers according to the invention, depending on the fluidity desired for an application generally carried out by injection in wells or in transport lines.
The preparation of the compounds of the present invention and the effects that can be obtained therefrom as to the improvement of the petroleum crudes will become more apparent from the non-limitative examples given herein-below. In order to facilitate reading, several definitions, measuring principles and usual tests methods in this field are set forth below.
The molecular weights of the polyacrylic acids utilized as intermediaries, or even as raw materials in the preparation of the compounds according to the invention are obtained from intrinsic viscosity values through application of the Mark-Houwink formula: EQU [n]=KM.sup.a
The viscosity measurements are taken on polyacrylic acid solutions in NaOH 2M, at 25.degree. C., and the values of the constants are taken from J. Banrup and E. H. Immergut, Polymer Handbook: EQU k=42.2 10.sup.-5 dl.g.sup.-1 EQU a=0.64
The molecular weights of the condensates according to the invention are determined by chromatography through gel permeation, as being those of polyethylene glycols that present the same chromatographic behaviour.
The paraffins contents of the experimented crudes is measured according to standard techniques of gas phase chromatography.
The pour point of the crudes is measured, according to the standard NFT 60105, but where necessary a dynamic pour test is also carried out. According to this test, the longest distance covered by an oil escaping from a vase through a cooled tube is measured. The oil is contained in a recipient maintained at a temperature of about 10.degree. higher than that of its pour point. This recipient issues by means of a largemouth cock, onto a calibrated glass tube, 40 cm in length, smaller than 3 mm in diameter, and inclined at 5.degree. on the horizontal. This tube is maintained, due to a thermostatic bath at a temperature of 0.degree., 5.degree., 10.degree. or 15.degree. -the choice of this temperature depening upon the performances of the oil and of the inhibitor subjected to the test. The initial level of the oil in the banking is relatively unimportant. Care has simply to be taken that it is maintained constant from one comparative test to another; in practice, the banking is charged with a constant volume of oil (for example 10 ml). The measurement is made by reading the maximum distance covered in the cooled tube prior to its solidification.
Furthermore, the aptitude of an additive to limit the formation of the paraffin deposit is measured by means of a test called plate test, which consists in circulating a crude, maintained at constant temperature, tangentially to two plates maintained at a different constant temperature to that of the crude, and which in any case is chosen lower than that of the starting crystallization temperature of the crude paraffins, itself determined, for example, by differential calorimetric analysis. After a certain period, the deposit formed is collected, weighed and possibly analyzed.